In 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), a ZC sequence is employed as an RS (reference signal) used in uplink. The reasons a ZC sequence is employed as an RS include constant frequency performance, good auto-correlation performance, good cross-correlation performance, and so on. A ZC sequence is a kind of a CAZAC sequence (Constant Amplitude and Zero Auto-correlation Code) and represented by following equation 1 or 2.
                    (                  Equation          ⁢                                          ⁢          1                )                                                                                  a            r                    ⁡                      (            k            )                          =                  {                                                                                          ⅇ                                                                  -                        j                                            ⁢                                                                        2                          ⁢                          π                          ⁢                                                                                                          ⁢                          u                                                N                                            ⁢                                              (                                                                                                            k                              2                                                        /                            2                                                    +                          qk                                                )                                                                              ,                                      N                    ⁢                                          :                                        ⁢                                                                                  ⁢                    even                                                                                                                                            ⅇ                                                                  -                        j                                            ⁢                                                                        2                          ⁢                          π                          ⁢                                                                                                          ⁢                          u                                                N                                            ⁢                                              (                                                                                                            k                              ⁡                                                              (                                                                  k                                  +                                  1                                                                )                                                                                      /                            2                                                    +                          qk                                                )                                                                              ,                                      N                    ⁢                                          :                                        ⁢                                                                                  ⁢                    odd                                                                                                          [        1        ]                                (                  Equation          ⁢                                          ⁢          2                )                                                                                  a            r                    ⁡                      (            k            )                          =                  {                                                                                          ⅇ                                          j                      ⁢                                                                        2                          ⁢                          π                          ⁢                                                                                                          ⁢                          u                                                N                                            ⁢                                              (                                                                                                            k                              2                                                        /                            2                                                    +                          qk                                                )                                                                              ,                                      N                    ⁢                                          :                                        ⁢                                                                                  ⁢                    even                                                                                                                                            ⅇ                                          j                      ⁢                                                                        2                          ⁢                          π                          ⁢                                                                                                          ⁢                          u                                                N                                            ⁢                                              (                                                                                                            k                              ⁡                                                              (                                                                  k                                  +                                  1                                                                )                                                                                      /                            2                                                    +                          qk                                                )                                                                              ,                                      N                    ⁢                                          :                                        ⁢                                                                                  ⁢                    odd                                                                                                          [        2        ]            In equations 1 and 2, N is the sequence length and u is the ZC sequence index, and N and u are coprime. Further, q is an arbitrary integer.
Generally, N−1 quasi-orthogonal sequences with good cross-correlation characteristics can be generated from a ZC sequence of sequence length N of a prime number. In this case, the cross-correlation between generated N−1 quasi-orthogonal sequences is constant at vN.
Amongst RSs used in uplink, a reference signal for channel estimation used to demodulate data (hereinafter “DM-RS,” which stands for demodulation reference signal) is transmitted in the same band as the data transmission bandwidth. That is, when the data transmission bandwidth is narrow, a DM-RS is transmitted in a narrow band, and, when the data transmission bandwidth is wide, a DM-RS is transmitted in a wide band. For example, if the data transmission bandwidth is one RB (resource block), the DM-RS transmission bandwidth is also one RB, and, if the data transmission bandwidth is two RBs, the DM-RS transmission bandwidth is also two RBs.
In 3GPP LTE, one RB is formed with twelve subcarriers, so that the number of subcarriers forming a transmission bandwidth is an integral multiple of twelve. Further, 3GPP LTE determines to use ZC sequences in a transmission bandwidth of 3 RBs or more. Based on this, a DM-RS transmitted in 3 RBs uses a ZC sequence of sequence length N of 31, and a DM-RS transmitted in 4 RBs uses a ZC sequence of sequence length N of 47. Then, the ZC sequence of sequence length N of 31 and the ZC sequence of sequence length N of 47 are individually subjected to cyclic extension (i.e. forward data of the sequences is copied and attached to the rear of the sequences), to generate DM-RSs having 36 subcarriers and 48 subcarriers.
As the method of ZC sequence allocation, by allocating ZC sequences with different sequence indices as DM-RSs in RBs, interference between DM-RSs used in different cells, that is, reduce inter-cell interference between DM-RSs is reduced. The data transmission bandwidth is determined based on scheduling of cells, so that DM-RSs in different transmission bandwidths are multiplexed between cells. When ZC sequences in different transmission bandwidths, that is, ZC sequences with different sequence lengths, are multiplexed, cross-correlation increases in a given combination of ZC sequence indices.
FIG. 1 shows cross-correlation characteristics between ZC sequences in different combinations of sequence indices. To be more specific, FIG. 1 shows cross-correlation characteristics between ZC sequence of sequence length N=31 and sequence index u=1 and ZC sequences of sequence length N=59 and sequence indices u=1 to 6. In FIG. 1, the horizontal axis shows delay time using the number of symbols, the lateral axis shows a normalized cross-correlation value (the value obtained by dividing a cross-correlation value by signal energy). As shown in FIG. 1, in the combination of the ZC sequence of N=31 and u=1 and the ZC sequence of N=59 and u=2, the maximum cross-correlation value increases significantly, and the cross-correlation value is about five times the cross-correlation value in the same transmission bandwidth, 1/vN, that is, 1/v31.
FIG. 2 shows inter-cell interference of DM-RSs when specific ZC sequence combinations of significant cross-correlation are allocated to neighboring cells. Specifically, a ZC sequence of N=31 and u=a and a ZC sequence of N=59 and u=b are allocated to cell #A and a ZC sequence of N=59 and u=c and a ZC sequence of N=31 and a=d are allocated to cell #B. In this case, by the combination of the ZC sequence of N=31 and u=a allocated to cell #A and the ZC sequence of N=59 and u=c allocated to cell #B, or by the combination of the ZC sequence of N=59 and u=c and the ZC sequence of N=31 and u=d allocated to cell #B, inter-cell interference of DM-RSs increases, the accuracy of channel estimation deteriorates, and therefore the performance of data demodulation significantly deteriorates.
Then, in cellular radio communication systems, the method of ZC sequence allocation disclosed in Non-Patent Document 1 is employed. Non-Patent Document 1 proposes allocating a combination of ZC sequences with significant cross-correlation and with different sequence lengths, to the same cell, in order to reduce inter-cell interference.
FIG. 3 illustrates the method of ZC sequence allocation disclosed in Non-Patent Documents 1 and 2. FIG. 3 uses an example shown in FIG. 2. As shown in FIG. 3, one combination of ZC sequences of significant cross-correlation, that is, the combination of a ZC sequence of N=31 and u=a and a ZC sequence of N=59 and u=c is allocated to the same cell (here, cell #A). Further, the other combination of ZC sequences of significant cross-correlation, that is, the combination of a ZC sequence with N=31 and a=d and the ZC sequence with N=59 and u=b is allocated to the same cell (here, cell #B). In a cell, one base station schedules transmission bands, and therefore, ZC sequences of significant cross-correlation allocated to the same cell are not multiplexed. Consequently, inter-cell interference is reduced.
Further, Non-Patent Document 2 proposes a method of finding groups of ZC sequence indices (hereinafter “sequence groups”) used in RBs. A ZC sequence has one characteristic of having higher cross-correlation when the difference in u/N is smaller. Then, the ZC sequences showing the difference in u/N equal to or less than a predetermined threshold value are found with reference to a sequence of given RBs (e.g. 3 RBs), from ZC sequences in RBs, and a plurality of ZC sequences as one sequence group are allocated to a cell.
According to the method of sequence group generation disclosed in Non-Patent Document 2, first, sequence length Nb and sequence index ub as a reference are set. Hereinafter, a ZC sequence with reference sequence length Nb and reference sequence index ub is referred to as a “reference sequence.” If Nb=31 (the sequence length corresponding to 3 RBs) and ub=1 (arbitrarily selected from 1 to Nb−1), ub/Nb=1/31. Next, ZC sequences showing the difference from reference ub/Nb in u/N equal to or less than a predetermined threshold value are found from ZC sequences in each RB, to generate a sequence group. Further, other sequence groups are generated in the same steps by changing a sequence index as the reference. In this way, it is possible to generate sequence groups equaling the number of sequence indices as references, that is, generate Nb−1 different sequence groups.
Here, assuming that the ZC sequences showing the difference from ub/Nb equal to or less than the predetermined threshold value overlap between neighboring sequence groups, the same ZC sequences are included in a plurality of sequence groups, and sequence indices collide between cells. Then, to prevent ZC sequences in neighboring sequence groups from overlapping, the above predetermined threshold value is set up with a smaller value than 1/(2Nb), for example.
FIG. 4 shows sequence groups generated by the method of sequence group generation disclosed in Non-Patent Document 2. Here, sequence length N is set up smaller than the size that can be transmitted in the transmission bandwidth and is set up the closest prime number to this size, and is uniquely found from the number of RBs. FIG. 4 shows sequence groups formed with ZC sequences satisfying following equation 3 in a case where the reference sequence length is Nb=31 and reference sequence indices are ub=1 to 30. In equation 3, threshold value Xth is, for example, Xth=1/(2Nb)=1/62 so that the same sequence is not included in a plurality of sequence groups.|ub/Nb−u/N|=Xth  (Equation 3)
Further, Non-Patent Document 3 discloses the relationships between the differences in u/N between ZC sequences and cross-correlation values of those ZC sequences, as shown in FIG. 5. FIG. 5 shows that, when the difference in u/N is close to 0, cross-correlation between sequences becomes the greatest and when the difference in u/N is close to 0.5, the cross-correlation becomes the second greatest.    Non-Patent Document 1: Huawei, R1-070367, “Sequence Allocation Method for E-UTRA Uplink Reference Signal,” 3GPP TSG RAN WG1 Meeting #47bis, Sorrento, Italy 15-19 January, 2007    Non-Patent Document 2: LG Electronics, R1-071542, “Binding method for UL RS sequence with different lengths,” 3GPP TSG RAN WG1 Meeting #48bis, St. Julians Malta, Mar. 26-30, 2007    Non-Patent Document 3: Panasonic, R1-074397, “Further consideration on uplink RS hopping and grouping,” 3GPP TSG RAN WG1 Meeting #50bis, Shanghai, China, Oct. 8-12, 2007